The present invention relates to a shadowmaskless tracking cathode ray tube control circuit comprising a control loop having a loop gain control means, which control loop is coupled to the shadowmaskless tracking cathode ray tube.
Such a shadowmaskless tracking cathode ray tube control circuit is known from GB-A-1 403 061. Disclosed therein is a shadowmaskless tracking cathode ray tube (CRT, also referred to as index tube) wherein a feedback control loop controls the exact landing spot of electron beams on a front plate thereof. The position of the spot is measured by using a conducting track structure or a sensor positioned on the inner side of the front plate. When a spot is scanned along the tracks, a tracking signal which depends on the spot position is produced, which tracking signal can be used to generate, in a loop, a spot position output signal for a deflection unit on the shadowmaskless tracking CRT, which keeps the spot on track. It has been found that the accuracy of the measurement is proportional to the loop gain and that its stability is inversely proportional to the loop gain. The loop gain depends on the beam current. This means that the loop gain has to be chosen in such a way that the stability will be sufficient in light areas on the screen. This is, however, disadvantageous for the accuracy in darker areas on the screen.
Therefore, it is an object of the present invention to provide a control loop circuit having an enhanced loop stability/accuracy balance under various light and dark circumstances on the CRT screen.
To this end, the shadowmaskless tracking cathode ray tube control circuit according to the invention is characterized in that the loop gain control means is arranged to compensate the loop gain of the control loop in dependence on the inverse value of a beam current in the cathode ray tube.
A compensation as thus proposed in the shadowmaskless tracking CRT circuit according to the present invention advantageously reveals a constant loop gain, which is independent of the beam current. The loop gain may at wish be optimized for improved stability and/or improved accuracy of the shadowmaskless tracking cathode ray tube control circuit according to the invention.
An embodiment of the shadowmaskless tracking CRT circuit according to the invention, which is easy to implement is characterized in that the loop gain control means comprises an arithmetic means.
A further embodiment of the CRT circuit according to the invention is characterized in that the arithmetic means comprises a multiplier. The multiplier thus multiplies the tracking signal by the inverse value of the beam modulation signal, which beam modulation signal is the video signal.
A still further embodiment of the CRT circuit according to the invention is characterized in that the arithmetic means comprises a normalizer unit.
Contrary to the previously mentioned further embodiment, this still further embodiment does not use the video signal, which comes from outside the loop, inside the loop. Instead, it arithmetically manipulates its signals, in particular current signals in order to calculate an inverse beam modulation signal to reveal the beam modulation independent loop gain. Advantageously, the arithmetic calculations do not involve time and frequency-dependent features of said signals.
Another embodiment of the shadowmaskless tracking CRT control circuit is characterized in that the normalizer unit comprises an adder, a subtracter and/or a divider. These are components, which are easy to implement.
The present invention also relates to an image device, such as a monitor or television device comprising a shadowmaskless tracking cathode ray tube control circuit as defined in claims 1-5, in which the shadowmaskless tracking cathode ray tube control circuit comprises a control loop having a loop gain control means, which control loop is coupled to a shadowmaskless tracking cathode ray tube, said device being characterized in that the loop gain control means is arranged to compensate the loop gain of the control loop in dependence on the inverse value of a beam current in the shadowmaskless tracking cathode ray tube.
In addition, the present invention relates to a method of controlling the spot position in a shadowmaskless tracking cathode ray tube, which method is characterized in that the spot position is controlled in such a way that the control takes the inverse value of the beam current in the shadowmaskless tracking cathode ray tube into account.